1. Technical Field
The present invention relates to a piston for a spark-ignited cylinder fuel injection engine and, more specifically, to the piston having improved ignitability and combustion stability with respect to an air-fuel mixture.
2. Related Art
For example, in a spark-ignited cylinder fuel injection (direct injection) engine used as a travelling power source for a vehicle, improvements in ignitability and combustion stability of an air-fuel mixture are required. In order to achieve these improvements, forming an air-fuel-mixture rich area in a periphery of an ignition plug is required at the time of ignition, which enables desirable stratified charge combustion.
In the related art, in order to form such an air-fuel-mixture rich area (layer), various methods are proposed. These methods include providing a crown surface of a piston with an uneven shape which enables swirling up of fuel sprays that are injected from an injector toward the crown surface and guiding the swirled fuel sprays toward the ignition plug.
In order to achieve both a stratified charge combustion performance and an uniform combustion performance concurrently, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 10-317973 describes a piston that is assembled to a pentroof type combustion chamber having two each of intake and exhaust valves. The piston includes a pair of intake valve recesses and a substantially true circle cavity combustion chamber that is formed at a central portion of the pair of intake valve recesses.
In order to expand an operating range that allows stratified charge combustion, JP-A No. 2000-130171 describes formation of a bowl stretching along a diameter line of a piston, which is orthogonal to a fuel spray axis, at a central portion of a crown surface of the piston, and a recess which is recessed further from a bowl surface and extends from an injector side to the bowl.
In order to increase a period that allows ignition at the time of stratified charge combustion, JP-A No. 2002-295261 describes formation of a recess at a central portion of a crown surface of a piston and a shelf having a smaller depth at a portion of the recess opposite to an injector.
In order to improve ignitability in a low load range, JP-A No. 2006-257943 describes formation of a recess having a substantially egg shape when seen in a direction of a cylinder axis on a crown surface of a piston.
In order to achieve combustion stability at the time of ignition retard without providing a deep cavity on a piston, JP-A No. 2008-151020 describes provision of a first step lower on an intake side and higher on an exhaust side on a crown surface and a second step provided beside the first step and being higher on an outer peripheral side of a cylinder under an ignition plug.
JP-A No. 2010-96088 describes formation of a groove-like recess stretching from a central portion of a crown surface to an outer peripheral surface of a piston in the vicinity of an injector. The recess has a depth increasing as it approaches a central portion of the piston.
JP-A No. 2013-113119 describes formation of a trapezoidal shaped cavity having a width decreasing gradually from an injector side toward a central portion of a piston.
JP-A No. 2013-113120 describes formation of a trapezoidal shaped cavity having a width increasing gradually from an injector side toward a central portion of a piston.
The aforesaid cylinder fuel injection engines that include the ignition plug disposed in the vicinity of the axial center of the cylinder and the injector disposed at a position offset from the ignition plug so as to inject fuel toward the town surface of the piston are desired to have improved ignitability and combustion stability with respect to the air-fuel mixture by making an air-fuel ratio in the periphery of the ignition plug richer and making an air-fuel-ratio rich area larger.